1. Field of the Invention
The present invention relates to an electronic interface device between a sensor sensing a physical magnitude, with analog output, and a display device displaying the value of said physical magnitude, including:
a controllable generator for controlling said display means and having an analog output, and
an analog amplitude comparator for comparing the amplitudes of the output signals of said sensor and of said controllable generator and whose binary output signal controls said controllable generator.
Such a device is used in particular on board motor vehicles. For example, such a device is inserted between the fuel level sensor in the tank of the vehicle and a display unit mounted on its dashboard.
2. Description of the Prior Art
A device of the above defined type is already known from the patent U.S. Pat. No. 3,983,549.
In this device, the controllable generator includes an oscillator, a shaping circuit, a counter, a D-A converter, and a memory. This controllable generator is adapted for delivering a monotonically increasing signal, and it is provided with an inhibition input, here in the memory, for "freezing" the output signal of the controllable generator when its value is equal to that of the output signal of the sensor. The assembly is controlled periodically for sampling the output signal of the sensor so as to obtain, on the display unit, a succession of values equal to the successive samples of the output signal of the sensor.
Such a device has however the drawback that it provides no damping of the rapid parasite variations which may effect the physical magnitude. In the case where this physical magnitude is the level of fuel in the tank of a vehicle, the rapid variations of the output signal of the sensor, which result from the haphazard movement of the fuel in the tank when the vehicle is moving, are not damped and so a stable indication is not obtained on the display unit representative of the mean level in the tank. So as to avoid supplying false indications when the vehicle is slanted, the device of patent U.S. Pat. No. 3,983,549 is adapted so that updating of the display is inhibited when the vehicle is on a slant, bu this provides no averaging effect of the signal at the output of the sensor and therefore does not guarantee that the displayed value represents the mean level of the fuel.
An interface device providing averaging is moreover known and described in the french application No. 85 19396 in the name of the applicant. In this device, the rapid parasite variations of the output signal of the sensor are damped by a low pass analog filter, using a resistor and a capacitor. Taking into account the fact that the time constant of the filter used must be relatively high, it is necessary to use a large capacitor, so of high cost price and space wasting. In addition, if it is desired to temporarily modify the filtering paramters so as to obtain a more satisfactory operation of the device, by reducing for example the time constant in the moments following switching off of the vehicle, so that the display unit gives an exact indication rapidly, it is necessary to use relays which further increase the cost and space required by the device.
To avoid these drawbacks, the above analog filter may be replaced by a digital filter, not requiring any filtering capacitor and of greater flexibility in use. This is the case with the devices described in the British application GB-A No. 2 100 487 and the German application DE-A No. 2 849 066, which include at the input an A-D converter, at the output a D-A converter and, between the two, several digital comparators and numerous digital processing circuits. The disadvantage of these devices is that they are complex and so require either numerous components, or a considerable semiconductor area if the assembly is formed on a single substrate.
The present invention overcomes the above drawbacks by providing a device capable of damping the rapid parasite variations of the physical magnitude by an averaging effect, and using neither high value capacitors nor complex digital processing circuits.